Filial Products 579 



partially separated another radio-active element which he called 

 actinium. It gives rise to an intermediate product actinium-X, 

 which yields an emanation with the short half-life of 3 P 9 seconds. 

 The emanation deposits two successive disintegration products ac- 

 tinium-A and actinium-B. 



Evidence gradually accumulated that the amounts of actinium in 

 radio-active minerals were, roughly at any rate, proportional to the 

 amounts of uranium. This result pointed to a lineal connection 

 between them, and led Boltwood to undertake a direct attack on the 

 problem. Separating a quantity of actinium from a kilogramme of 

 ore, Boltwood observed a growth of 8 '5 x 10~ 9 gramme of radium in 

 193 days, agreeing with that indicated by theory within the limits 

 of experimental error 1 . We may therefore insert provisionally 

 actinium and its series of derivatives between uranium and radium 

 in the radio-active pedigree. 



Turning to the other end of the radium series we are led to ask 

 what becomes of radium-F when in turn it disintegrates? What is 

 the final non-active product of the series of changes we have traced 

 from uranium through actinium and radium ? 



One such product has been indicated above. The a-ray particles 

 appear to possess the mass of helium atoms, and the growth of helium 

 has been detected by its spectrum in a tube of radium emanation. 

 Moreover, helium is found occluded in most if not all radio-active 

 minerals in amount which approaches, but never exceeds, the 

 quantity suggested by theory. We may safely regard such helium 

 as formed by the accumulation of a-ray particles given out by succes- 

 sive radio-active changes. 



In considering the nature of the residue left after the expulsion 

 of the five a-particles, and the consequent passage of radium to 

 radium-F we are faced by the fact that lead is a general constituent 

 of uranium minerals. Five a-particles, each of atomic weight 4, 

 taken from the atomic weight (about 225) of radium gives 205 a 

 number agreeing fairly well with the 207 of lead. Since lead is more 

 permanent than uranium, it must steadily accumulate, no radio-active 

 equilibrium will be reached, and the amount of lead will depend on 

 the age of the mineral as well as on the quantity of uranium present 

 in it. In primary minerals from the same locality, Boltwood has 

 shown that the contents of lead are proportional to the amounts of 

 uranium, while, accepting this theory, the age of minerals with a given 

 content of uranium may be calculated from the amount of lead they 

 contain. The results vary from 400 to 2000 million years 2 . 



1 American Journal of Science, December, 1906. 



2 American Journal of Science, October, 1905, and February, 1907. 



372 



